GK104: 3 different types?

I was looking at TechPowerUp's GPU database and noticed there were three different types of GK104, all w/differing numbers of shaders, ROP's and TMU's.

How can you have one chip fab produce three ASIC's that are different? Or are they all manufactured the same and then modified, post-production somehow?

Is it possible Nvidia is uses the same trick AMD did with the 6950/6970 and uses a BIOS hack to disable shaders, ROP's and TMU's?

That's how both NVIDIA and AMD make their GPUs. They make the full-fledged one, then just cut it down multiple times to fill out their lineup of GPUs. Sometimes shaders are unlockable, but that's typically not the case.